1. Field of the Invention
The present invention relates to a projection exposure apparatus which is suitable to be applied to a projection exposure apparatus of a type in which a wafer stage is driven in a step-and-repeat method.
2. Related Background Art
A prior art projection exposure apparatus is shown in FIG. 4.
A photo-mask (hereinafter referred to as a reticle) 5 is supported by a reticle support member 4 and kept in a conjugate positional relation with a photo-sensitive substrate (hereinafter referred to as a wafer) 2 mounted on a wafer stage 1, through a projection lens 3.
A movable mirror 7 is attached to an end of the wafer stage 1 and it is moved as the wafer stage 1 is moved. A fixed mirror 6 is mounted in a lens barrel of the projection lens 3.
The movable mirror 7 reflects a laser beam directed from a laser beam interference range finder (interferometer) 10 through a beam splitter 9, and the fixed mirror 6 reflects a laser beam directed from the interferometer 10 through the beam splitter 9 and a reflection mirror 8. The interferometer 10 measures the reflected light from the movable mirror 7 and the fixed mirror 6 to detect a relative position of the movable mirror 7 and the fixed mirror 6.
A control unit 11 controls the drive of the wafer stage 1 to a desired position by a drive unit 12 in accordance with the relative position of the movable mirror 7 and the fixed mirror 6 detected by the interferometer 10.
In such a type of projection exposure apparatus, the reticle support member 4, the projection lens 3 and the wafer stage 1 are integrally coupled together by a predetermined support member (not shown). Thus, when the stage 1 is driven in X and Y directions, the entire apparatus is vibrated in such a direction that an optical axis AX is inclined around the wafer stage 1, as shown by a broken line in FIG. 4.
In this case, it has been considered that there is no positional shift of the reticle 5 and the wafer 2 because the relative position of the reticle 5, the projection lens 3 and the wafer 2 does not change like a pendulum.
However, because the wafer stage 1 is movable in X and Y directions, a center of gravity changes by the movement. When a vibration is generated, a rotation error component to the reticle 5 and the wafer stage 1 is included in a vibration component due to a twist of the entire apparatus or stage yawing. Further, a shift component or a rotation component to the reticle 5 and the wafer stage 1 is included in the vibration component due to a delay in transmission of the vibration and a difference between lateral shifts by the vibration between the fixed mirror 6 and the reticle 5. By the inclusion of the rotation error component and the shift component (or rotation component) in the vibration component, the relative position of the reticle 5 and the wafer 2 varies, which causes a stepping error. As a result, precise exposure is not attained.